Py-trees

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* [meta] add children monkeypatching for composite imposters
* [blackboard] check for nested variables in WaitForBlackboard

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* [docs] bugfix image links and rewrite the motivation

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* [infra] setup.py tests_require, not test_require

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* [infra] update maintainer email

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* [docs] many minor doc updates
* [meta] bugfix so that imposter now ticks over composite children
* [trees] method for getting the tip of the tree
* [programs] py-trees-render program added

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* [infra] handle pypi/catkin conflicts with install_requires